This invention relates to digital communication over two-wire twisted pair telephone lines in the environment of a local area network, and more particularly this invention relates to the transmission of signals in accordance with IEEE Standard 802.3 (Ethernet) signals of a bandwidth in the range of d.c. to greater than about 4 MHz and preferably greater than about 10 MHz and less than about 20 MHz over an unshielded 24 gauge typical twisted pair of telephone wires within the walls of an office.
One of the challenges of office communications is to be able to communicate data signals from d.c. to over 10 MHz data rate through available office wiring without signal reflections. Passing unbalanced signals at d.c. and low frequencies is particularly challenging in a broad bandwidth medium. What is therefore needed is an apparatus which provides signal matching of unbalanced signals to balanced signals to allow runs of twisted pair wires to replace runs of coaxial cable in an office environment.
Prior proposed solutions to the problem of transmitting signals over telephone-type twisted pair wiring in the speed range of interest (over 1 MBS) have involved expensive construction with active components or non-bus approaches (STARLAN, IBM Token Ring on baseband twisted pair, Synoptics ASTRANET, ONAN Type 3 Media filter for IBM Token Ring on baseband twisted pair, MUX LABS 3270 Coax Replacements) in order to avoid the problem and difficulties of measuring high frequency impedance as well as the problem of matching disparate impedances of various types of wiring. The MUX LABS 3270 Coax Replacement (from MUX LABS of Montreal, Quebec, Canada) employs a simple one-to-one isolation transformer which is incapable of passing d.c. This device is intended only for use in point-to-point connections. The ONAN Type 3 Media Filter (from ONAN Power Electronics of Minneapolis, Minnesota provides a rudimentary low-pass filter and common mode rejection transformers for a 4 MBS Token Ring (IEEE 802.5) System. However, the ONAN device was apparently designed with certain assumptions which do not apply to the matching of twisted pair wiring to coaxial cable. For example, the design appears to assume balanced connections and an impedance match at the transmit terminal of the ring. What is needed is a simple, low-cost bus-type (single-pair) wiring solution for a local area network application capable of matching IEEE Standard 802.3 (Ethernet) 50-ohm unbalanced signals to a substantially balanced pair of telephone type wires of unknown impedance in the 50-ohm to 200-ohm range.